Apparatus And Method For Aligning A Wafer

ABSTRACT

A method for aligning a wafer on a support member within a vacuum chamber includes increasing the pressure within the vacuum chamber to at least about 1 Torr before aligning the wafer. The wafer is introduced into the vacuum chamber on the support member, the pressure is increased to at least about one Torr, and the support member is lifted into a shadow ring that has a frustoconical inner cavity constructed to funnel the wafer to a centered, aligned position.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to the field of semiconductor waferprocessing equipment. More particularly, the present invention relatesto a method and apparatus for aligning a wafer on a wafer supportmember.

[0003] 2. Background of the Related Art

[0004] In the fabrication of integrated circuits, the various processes,such as physical vapor deposition (PVD), chemical vapor deposition(CVD), and etch processes, are often carried out in a vacuum environmentto, among other things, reduce the particulate level to which the wafersare exposed. Wafers are introduced into a vacuum processing systemthrough a loadlock where robots within the vacuum processing system movethe wafers from the loadlock into a transfer chamber and thensequentially through the system positioning the wafers in a series ofprocessing chambers.

[0005] The processing steps carried out within the vacuum chamberstypically require the deposition, or etching of multiple metal,dielectric and semiconductor film layers on the surface of a wafer.During these processing steps, one must properly align and secure thewafer in the processing chamber in which the desired deposition or etchprocess is performed.

[0006] Typically, the wafer is supported in the chamber on a supportmember, commonly called a susceptor or pedestal. The wafer is placed onor secured to, the upper surface of the support member prior to thedeposition or etch process. To ensure proper processing of the wafer,the wafer must be properly aligned relative to the support member. Theposition of the support member in the chamber is selected to provide adesired spacing and relative geometry between the generally planarsurface of the wafer and other portions of the process chamber such as agas plate in a CVD process or a target in a PVD process.

[0007] Generally, a shadow or clamp ring is used to shield the edge of awafer and/or, in the case of a clamp ring, secure the wafer to thesupport member. Although the present invention is equally applicable toboth shadow rings and clamp rings, the following description will referprimarily to shadow rings such as those typically used in CVD processes.In addition to acting as a shield, shadow rings also function in wafercapturing or alignment on the support member. Wing members extenddownwardly and outwardly from the shadow ring to form a funnel. As thesupport member moves the wafer upward into the processing position, thesupport member moves the wafer into the funnel which directs the waferinto alignment with the shadow ring and the support member.Consequently, the funnel applies vertical and lateral forces to thewafer when the slanted wing members achieve lateral alignment of amisaligned wafer with the shadow ring and support member as the supportmember moves the wafer to the top end of the funnel and the shadow ringsettles on the support member.

[0008] A primary goal of wafer processing is to obtain as many usefuldie as possible from each wafer. Many factors influence the processingof wafers in the chamber and effect the ultimate yield of die from eachwafer processed therein including the existence of contaminants withinthe chamber that can attach to the wafer and contaminate one or more dietherein. The processing chambers have many sources of particlecontaminants which, if received on the wafer, reduce the die yield. Onesource of particulate contamination occurs when a misaligned wafer isintroduced into the chamber. As the wing members of the shadow ringalign with the wafer, the wafer slides on the flat surface of thesupport member and, due to the frictional forces between the wafer andthe support member, may create particulate contaminants. In some cases,the frictional forces between the wafer and the support member cause themisaligned wafer to actually move the shadow ring, thereby preventingproper alignment of the wafer and reducing repeatability of the zone ofexclusion shielded by the shadow ring and the process.

[0009] Prior efforts aimed at reducing the creation of particles havereduced the alignment movement of the wafer on the support member andsimply increased the amount of overhang by the shadow ring. In this way,the shadow ring is able to cover the wafer without substantial movementof the wafer. One way that this is accomplished is by increasing thediameter of the shadow ring funnel upper end so that this diameter islarger relative to the diameter of the wafer and the support member.Thus, rather than substantially moving the wafers to align them, thesesystems simply accept a greater misalignment and accept greater coverageof the wafer upper surface area.

[0010] However, a second factor influencing the processing of wafers inthe chamber and affecting the ultimate yield of die from each waferprocessed therein is the repeatability of the positioning of the waferand the area covered by the shadow ring. The wafer must be properlyaligned relative to the support member and the shadow ring to ensurethat the film is properly deposited on the wafer. Therefore, these priorefforts that avoid alignment of the wafer and cover more surface areaare not acceptable.

[0011] It would, therefore, be desirable to provide a relatively simplesystem and method for reducing the coefficient of friction between thesupport member and the wafer that would allow alignment of the waferwithout substantial particle generation.

SUMMARY OF THE INVENTION

[0012] In view of the foregoing, it is an object of the invention toprovide a relatively simple apparatus and method for reducing thefrictional forces between the support member and the wafer. It isanother object of the invention to enhance repeatability and to providea shadow ring that covers a minimal area of the upper surface of thewafer. Yet another object of the invention is to provide a system andmethod for aligning a wafer that is relatively inexpensive, efficient,simple to implement, and reliable. Other objects of the invention willbecome apparent from time to time throughout the specification andclaims as hereinafter related.

[0013] The present invention provides methods and apparatuses foraligning a wafer on a support member in a vacuum chamber. In one aspectof the invention, the method comprises the steps of introducing thewafer into the vacuum chamber, increasing the pressure within the vacuumchamber and moving the wafer into alignment with a support member and/orshadow ring.

[0014] In another aspect, the method comprises providing a shadow ringhaving a lower portion that is outwardly tapered for receipt of a waferand an upper aperture having a diameter that is slightly less than theouter diameter of the wafer, introducing the wafer into the vacuumchamber and onto the support member, increasing the pressure within thechamber, and subsequently moving the support member towards the shadowring so that the shadow ring aligns the wafer on the support member.

[0015] In accordance with the methods, the apparatus for aligning awafer on a support member in a vacuum chamber is an apparatus comprisinga support member positioned within the vacuum enclosure and having awafer receiving surface thereon, a shadow ring located within the vacuumchamber, a gas supply in fluid communication with the vacuum chamber,and a gas flow controller that controls the flow of gas to the vacuumchamber and, thereby, regulates the pressure within the vacuum chambersuch that, after the wafer is positioned on the support member andbefore the wafer is raised into the shadow ring, the control memberraises the pressure within the chamber to about 1 Torr. The shadow ringused in this apparatus comprises an upper shield portion defining acircular aperture therethrough, the circular aperture having a diameterthat is slightly less than the outer diameter of the wafer, a lowerportion extending from the upper shield portion having an annular crosssection defining a frustoconical inner cavity, the diameter of the innercavity decreases from a lower mouth aperture to an upper end, and thediameter of the upper end of the inner cavity is slightly greater thanthe outer diameter of the wafer.

[0016] In each of these methods and apparatuses, the pressure ispreferably raised to a pressure greater than about 1 Torr and morepreferably to a pressure between about 1 Torr and 100 Torr and mostpreferably between about 1 Torr and 10 Torr. Further, the pressure israised is to approximately equal to or less than the process pressure.Also, the pressure between the wafer and the support member ispreferably equal to or greater than the pressure in the chamber beforethe wafer is aligned.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT

[0017] So that the manner in which the above recited features,advantages and objects of the present invention are attained and can beunderstood in detail, a more particular description of the invention,briefly summarized above, may be had by reference to the embodimentsthereof which are illustrated in the appended drawings.

[0018] It is to be noted, however, that the appended drawings illustrateonly typical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

[0019]FIG. 1 is a partial cross sectional view of the vacuum chamber.

[0020]FIG. 2 is a schematic drawing of the vacuum chamber and thepressure control system.

[0021]FIG. 3 is a cross sectional view of a typical support memberhaving a wafer thereon that is partially covered by a shadow ring.

[0022]FIG. 4 is a partial, cross sectional view of a shadow ring, awafer, and a support member showing the wafer misaligned on the supportmember as they enter the inner cavity of the shadow

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0023] As shown in FIG. 1, the present invention relates to a method andapparatus for aligning a wafer 20 on a support member 60 in a vacuumchamber 30. The alignment apparatus is depicted generally as 10.

[0024] The preferred embodiment described below refers to an alignmentapparatus 10 that uses a shadow ring 40 to align the wafer 20 on thesupport member 60. However, the invention is not limited to this preciseform of apparatus for it may apply to any number of alignmentmechanisms. As previously mentioned, the term “shadow ring,” as usedherein, refers generally to both shadow rings and clamp rings.

[0025]FIG. 1 shows a typical vacuum chamber 30 defined by an outer body32. The vacuum chamber 30 houses a support member 60 that may take theform of a pedestal or susceptor mounted on a generally verticallyoriented shaft 62. The support member 60 serves to support a wafer 20 onits flat upper supporting surface 64. The support member 60 alsoincludes a step formation 68 formed on its outer perimeter to receiveand support a shadow ring 40 and includes four finger apertures 66.

[0026] In a typical vacuum chamber 30, the pressure within the vacuumchamber 30 is controlled by a pressure control system such as the oneshown schematically in FIG. 2. In this system, a gas supply 170 isprovided in fluid communication with the vacuum chamber 30. A gas flowcontroller 180 positioned intermediate the gas supply 170 and the vacuumchamber 30 controls the flow from the gas supply 170 to the vacuumchamber 30. Using a predetermined set of instructions, the gas flowcontroller 180 selectively provides a flow of gas to the vacuum chamber30. As the gas flows into the vacuum chamber 30, the pressure within thevacuum chamber 30 increases. In this way, the gas flow controller 180controls the pressure within the vacuum chamber 30. It is possible toprovide the gas to the chamber 30 through the support member 60 to theback side of the wafer 20. When provided to the back side of the wafer20, the gas creates a pressure between the wafer 20 and the supportmember 60 that is initially greater than the pressure in the chamber 30.This back side gas may be provided, for example, by a bypass line 200that provides communication from the gas flow controller 180 to theupper surface 64 of the support member 60 between the support member 60and the wafer 20.

[0027]FIG. 1 also illustrates a wafer lifting finger 90 received in afinger aperture 66 passing through the body of the support member 60.Typically, the processing chamber would include four such liftingfingers 90. These lifting fingers 90 operate to lift the wafer 20 clearof the upper supporting surface 64 of the support member 60 afterprocessing. This removal of the wafer 20 is achieved by means of aconventional processing apparatus robot arm (not shown) which enters thevacuum chamber 30 through the slit valve opening 36. The same robot armis also used to insert the wafers 20 into the vacuum chamber 30. Thelifting fingers 90 are movable vertically under action of a liftingmechanism 92 of which only the upper portion is shown.

[0028] A shadow ring 40 housed within the vacuum chamber 30 operates toprovide an exclusionary zone where no deposition occurs at the edge ofthe wafer 20. The shadow ring 40 also operates to force a misalignedwafer 20 into alignment as the support member 30 moves from a lowered,or idle, position to a raised, or processing, position. When the supportmember 30 is in the lowered position, the shadow ring 40 is supportedaround its perimeter by an outer support ring 38 that is, in turn,supported by a conventional pumping plate 39 attached to the vacuumchamber 30. Together, the two rings, 40 and 38, divide the vacuumchamber 30 into upper and lower sections, 30 a and 30 b respectively.

[0029] During processing, the support member 60 moves upward into araised position lifting the shadow ring 40. The shadow ring 40 has alower portion 42 that rests on the upper surface 64 of the supportmember 60 and supports the upper shield portion 50 of the shadow ring 40above the upper surface of the wafer 20. Preferably, the shield portion50 is held about 5 to 10 mils above the wafer 20. The upper shieldportion 50 of the shadow ring 40 defines a circular upper aperture 46therethrough. The diameter of the upper aperture 46 may be slightly lessthan the outer diameter of the wafer 20 to form the exclusionary zone onthe wafer 20. However, new processes may require no overhang of theshadow ring 40 over the wafer 20. In one typical processing operation,the step formation 68, shown in FIG. 1, is in the range of 3.8 to 3.9 mmhigh, the shadow ring 40 is in the range of 5 to 5.1 mm thick, and theoverhanging portion is in the range of 0.8 to 0.9 mm thick. Theoverhanging portion defines an exclusionary zone of about 3 to 5 mmabout the edge of the wafer 20. However, in the preferred embodiment,this exclusionary zone is no greater than 1.5 mm from the edge of thewafer 20. To accommodate the current industry standards, theexclusionary zone at any one edge is preferably about 1.5 mm or less.This relatively small exclusionary zone is necessary to allow depositionon the wafer 20 at a position 1.5 mm from the wafer edge. Industrystandards demand a film thickness at 1.5 mm from the wafer edge that isat least 90 percent of the film thickness at the wafer center. Nodeposition is allowed on the beveled edge of the wafer 20. Therefore,for a typical wafer 20 having a 0.5 mm chamfer about its edge, thisallows a deviation of only about 1 mm from center. As used herein, alldimensions account for thermal expansion and are representative ofmeasurements at process temperatures.

[0030] Preferably, a purge gas is directed through the support member 60about the periphery of the wafer 20. The purge gas flows between theshadow ring 40 and the wafer 20 to help shield the exclusionary zone ofthe wafer 20.

[0031] A lower portion 42 of the shadow ring 40, as shown in FIG. 4,extends downwardly from the upper shield portion 50. The lower portion42 has an annular cross section throughout its length and defines afrustoconical inner cavity 44 therein that is concentric with the upperaperture 52. Because wafers 20 are circular in shape, the support member60 is circular as is the inner cavity cross section. The diameter of theinner cavity 44 decreases from the lower mouth portion 46 to the upperend 48 of the inner cavity 44 to form a funnel-like structure foraligning the wafer 20 on the support member 60. Accordingly, the surfaceof the inner cavity 44 is relatively smooth to facilitate the slidingreceipt and abutment of the wafer 20 in the inner cavity 44. To allowreceipt of the wafer within inner cavity 44 and to properly align thewafer 20 with the shadow ring 40, the diameter of the upper end 46 ofthe inner cavity 44 is slightly greater than and, preferably,approximately equal to the outer diameter of the wafer 20. As previouslymentioned, current industry practice demands that the thickness of thedeposited film at a position 1.5 mm from the edge of the wafer 20 to 90percent of the thickness at the center of the wafer 20. Accordingly, thewafer 20 must be aligned so that the shadow ring overhangs the wafer 20by no more than 1.5 mm about its full periphery so that the film will beallowed to deposit on the wafer 20 at 1.5 mm from the edge of the wafer20. Therefore, the diameter of the upper end 46 of the inner cavity 44is preferably at most only slightly more than 3 mm greater than theupper aperture 52 and only slightly greater than the outer diameter ofthe wafer 20 to ensure that the edge of the wafer 20 is within 1.5 mm ofthe periphery of the upper aperture 52. In this way, the shadow ring 40only overhangs the wafer 20 at most by about 1.5 mm about the fullperiphery of the wafer 20. Because the wafer 20 rests on the uppersurface 64 of the support member 60 and the wafer 20 is relatively thin,the outer diameter of the support member 60 must be sufficiently smallthat it can also be positioned proximal the upper end 52 of the innercavity 44. However, to provide proper support for the wafer 20, thesupport member 60 must cover substantially the full area of the wafer20. Therefore, the wafer must occupy most of the upper surface area ofthe support member 60.

[0032] As shown in FIG. 1, once positioned in the vacuum chamber 30, awafer 20 rests on the upper supporting surface 64 of the support member30. This placement is made with the support member 60 in its loweredposition. Before processing may begin, the wafer 20 must first be raisedby the support member 60 to the raised position. It is during themovement from the lowered position to the raised position that anymisalignment of the wafer 20 is corrected and the wafer 20 is aligned.As the support member 60 moves upward from the lowered position, amisaligned wafer 20 contacts the inner cavity 44 of the shadow ring 40at a position intermediate the upper end 48 and the lower mouth portion46. FIG. 4 illustrates a misaligned wafer 20 on the support member 60.The point of contact is dependent upon the magnitude of themisalignment. Preferably, there is no misalignment. As the supportmember 60 continues to move upward, the angled side of thefrustoconically-shaped inner cavity 44 exerts a lateral force on theedge of the wafer 20 forcing the wafer 20 into alignment. Consequently,when the support member 60 reaches its raised position so that the wafer20 is at the upper end 48 of the inner cavity 44 of the shadow ring 40,the wafer 20 is aligned due to the relative diameters of the wafer 20and the shadow ring components. When in this raised position, dependingupon the type of process involved, the outer portion of the wafer 20 mayeither bear against the shadow ring 40 and slightly lift the shadow ring40 under action of the support member 60 or may rest on the shoulder 68of the support member 60 and, thereby, leave a small gap between theshadow ring 40 and the wafer 20. For convenience, the application refersprimarily to those processes wherein the wafer 20 does not contact theshadow ring 40 although the present invention is applicable to allprocesses. With the support member 60 in the raised position, the outerportion of the wafer 20 is covered by the upper shield portion 50 of theshadow ring 40.

[0033] However, as mentioned previously, the sliding movement of thewafer 20 on the support member 60 during alignment creates particleswithin the vacuum chamber 30. These particles are generated as a resultof the friction between the wafer 20 and the support member 60 which isgenerally characterized by the coefficient of friction of the interfacemultiplied by the weight of the wafer 20. Other forces acting upon thewafer 20 also affect the magnitude of the frictional forces. Forexample, vacuum chucking may affect the friction between the wafer 20and the support member 60. Likewise, the downward component of the forceexerted by the frustoconical inner cavity 44 increases the frictionalforces between the abutting surfaces. Nevertheless, the friction forcebetween the surfaces equals the coefficient of friction between thesurfaces multiplied by the downward, normal forces exerted on the wafer20 whatever their source. Generally, the weights of the wafers 20 arerelatively constant. Greater frictional forces on the wafer 20 and thesupport surface 60 cause greater particle generation and decrease theenergy efficiency of the system. In addition, high frictional forces maycause misalignment and may cause the wafer 20 to move the shadow ring 40out of alignment, rather than the shadow ring 40 moving the wafer 20into alignment, if the lateral force applied on the wafer 20 by theshadow ring is insufficient to overcome the frictional forces. For thepurposes of the present application, the relevant normal and frictionalforces are generally characterized by the following formulasrespectively wherein N represents the normal force applied to the wafer20, F is the frictional force applied to the wafer 20, G is the weightof the wafer 20, A is the surface area of the wafer 20, P₁ is thepressure in the chamber 30, P₀ is the pressure between the wafer 20 andthe support member 60, and μ is the coefficient of friction.

N=G−(P ₁ −P ₀)A

F=μN=μ(G−(P ₁ −P ₀)A)

[0034] Thus, the normal force is equal to the weight of the wafer 20less the force created by the pressure differential on the top andbottom surfaces of the wafer 20. The force created by this pressuredifferential equals the difference between the pressure between thewafer 20 and the support member 60 and the pressure in the chamber 30multiplied by the surface area of the wafer 20. The frictional forcesequal the normal forces multiplied by the coefficient of friction.

[0035] Reducing the frictional forces between the wafer 20 and thesupport member 60 reduces the number of particles generated when thewafer 20 is moved on the support member 60. Accordingly, in order toreduce the number of particles generated, the coefficient of friction orthe normal force between the wafer 20 and the support member 60 must bereduced. The present invention accomplishes this by increasing thepressure within the vacuum chamber 30 to at least about one Torr.Empirical studies, which are more fully discussed below, have shown thatincreasing the pressure within the vacuum chamber 30, so that thepressure between the wafer 20 and the support member 60 is equal to orgreater than the pressure in the vacuum chamber 30, reduces thefrictional forces between the wafer 20 and the support member 60. Inorder for this decrease in frictional force to occur, one of two thingsmust happen. One possibility is that the increased pressure somehowlowers the coefficient of friction (e.g., by possibly creating a cushionof gas between the wafer 20 and the support member 60). Anotherpossibility is that the increased pressure somehow lowers the normalforce on the wafer 20. Regardless of the manner in which increasing thepressure affects the frictional forces, the result is that thefrictional forces are reduced and, thus, the wafer 20 may be moved onthe support member 60 with less resistance and less particle generation.The resulting decrease in frictional force allows freer movement of thewafer 20 on the support member 60 and, thereby, reduces the resultingscratches and generated particles. Gas from the gas supply 170 isintroduced into the vacuum chamber 30 to increase the pressure therein.The gas may be introduced generally into the chamber 30 or through gasinlets positioned in the upper surface 64 of the support member 60. Itis in this latter case that the pressure below the wafer 20 is greaterthan the pressure above the wafer 20.

[0036] Therefore, the method of the present invention involvesincreasing the pressure within the vacuum chamber 30 to at least aboutone Torr before moving the wafer 20 on the support member 60 foralignment. Typically, the pressure within the vacuum chamber 30 when thewafer 20 is introduced therein is about one milliTorr or less. The waferis, thus, introduced into the vacuum chamber 30 onto the support member60 which is in a lowered position. The support member 60 is then raisedto the lower mouth aperture 46 of the shadow ring 40. However, beforeraising the support member 60 to the processing position the pressurewithin the vacuum chamber 30 is increased to at least about one Torr. Ofcourse, this step of increasing the pressure may take place at any timebefore the support member 60 is raised into the inner cavity 44 of theshadow ring 40. Preferably, the pressure is raised to between about 1Torr and 100 Torr or, more preferably, between about 1 Torr and 10 Torrand approximately equal to or less than the operating pressure of theprocess. The operating pressure of the process is the pressure at whichthe process, such as a chemical vapor deposition process, is carried outin the vacuum chamber 30. Also, before raising the support member 60 tothe raised position, the pressure between the wafer 20 and the supportmember 60 is provided so that the pressure between the wafer 20 and thesupport member 60 is approximately equal to or greater than the pressurein the vacuum chamber 30. Once the pressure in the vacuum chamber 30 issufficiently raised and the pressure beneath the wafer 20 is equalized,the support member 60 is raised to the raised, or processing, position.As previously discussed, when the support member 60 moves into theshadow ring 40, any misaligned wafer 20 will contact the angled sides ofthe inner cavity 44 which will force the wafer 20 into alignment. Afterthe support member 60 is in the raised position and the wafer 20 isaligned, the pressure within the vacuum chamber 30 may be altered asneeded.

[0037] As previously described, the pressure within the vacuum chamber30 is manipulated by a gas supply 170 and a gas flow controller 180. Inoperation, the gas flow controller 180 uses predetermined set ofinstructions to adjust the pressure within the vacuum chamber 30 asneeded. A vacuum pump 190, or series of vacuum pumps 190, are used toevacuate the vacuum chamber 30.

[0038] Example

[0039] This system has been tested to determine its effectiveness asfollows. A misaligned wafer 20 was positioned upon a support member 60in a vacuum chamber 30 and was raised from a lowered position to araised position. The test was conducted under vacuum conditions (i.e.,moving the wafer 20 without first increasing the pressure in thechamber) and under pressurized conditions (i.e., moving the wafer 20only after increasing the pressure in the chamber). When tested underpressurized conditions, the tests were conducted with both the pressurebeneath the wafer 20 equal to and greater than the pressure in thechamber 30. In both of these pressurized condition tests, the resultswere essentially the same. The wafers 20 were then inspected using aSURISCAN 6200 manufactured by Tencor Instruments to determine the numberof particles generated as a result of the wafer 20 moving on the supportmember 60. The results revealed that, without first increasing thepressure in the chamber, alignment of the wafer generated approximately50 to 200 particles when the wafer 20 contacted the shadow ring andapproximately 5000 backside particles. In addition, without firstincreasing the pressure in the chamber, the shadow ring 40 often movedwith the wafer 20 as the support member 60 lifted the shadow ring 40 dueto the frictional forces holding the wafer 20 to the support member 60.This resulted in a misaligned wafer 20 and reduced repeatability of theprocess. However, using the present invention, wherein the pressure israised to at least about one Torr before moving the wafer 20, themovement of the wafer 20 on the support member 60 generated onlyapproximately Twenty (20) particles when the wafer 20 contacted theshadow ring 40 and less than 2000 backside particles. Further, themisaligned wafer 20 moved on the support member 60 more readily and was,therefore, properly centered which increased repeatability of the edgeexclusion and the process.

[0040] While the foregoing is directed to the preferred embodiment ofthe present invention, other and further embodiments of the inventionmay be devised without departing from the basic scope thereof, and thescope thereof is determined by the claims which follow.

1. A method for aligning a wafer within a vacuum chamber, comprising thesteps of: introducing the wafer into the vacuum chamber; increasing thepressure within the vacuum chamber, and subsequently moving the waferinto alignment.
 2. The method of claim 1 wherein the pressure in thevacuum chamber when the wafer is introduced therein is 1 milliTorr orless.
 3. The method of claim 1 further comprising the step of increasingthe pressure in the vacuum chamber to at least about 1 Torr.
 4. Themethod of claim 1 further comprising the step of increasing the pressurein the vacuum chamber to a pressure that is at least about 1 Torr andless than an operating pressure of the chamber.
 5. The method of claim 1further comprising the step of increasing the pressure in the vacuumchamber to a pressure between about 1 Torr and 100 Torr.
 6. The methodof claim 1 further comprising the step of increasing the pressure in thevacuum chamber to a pressure between about 1 Torr and 10 Torr.
 7. Themethod of claim 1 further comprising the step of waiting until thepressure between the wafer and the support member is equal to or greaterthan the pressure in the vacuum chamber before aligning the wafer.
 8. Amethod for aligning a wafer on a support member within a vacuum chamber,comprising the steps of: providing a shadow ring having a lower portionthat is outwardly tapered for receipt of the wafer and an upper aperturehaving a diameter that is slightly less than the outer diameter of thewafer; introducing the wafer into the vacuum chamber and onto thesupport member; increasing the pressure within the chamber; andsubsequently moving the support member toward the shadow ring so thatthe shadow ring aligns the wafer on the support member.
 9. The method ofclaim 8 wherein the pressure in the vacuum chamber when the wafer isintroduced therein is 1 milliTorr or less.
 10. The method of claim 8further comprising the step of increasing the pressure in the vacuumchamber to a pressure at least about 1 Torr.
 11. The method of claim 8further comprising the step of increasing the pressure in the vacuumchamber to a pressure that is at least about 1 Torr and less than anoperating pressure of the chamber.
 12. The method of claim 8 furthercomprising the step of increasing the pressure in the vacuum chamber toa pressure between about 1 Torr and 100 Torr.
 13. The method of claim 8further comprising the step of increasing the pressure in the vacuumchamber to a pressure between about 1 Torr and 10 Torr.
 14. The methodof claim 8 further comprising the step of waiting until the pressurebeneath the wafer is equal to or greater than the pressure in the vacuumchamber before aligning the wafer.
 15. The method of claim 8 furthercomprising the step of raising the wafer to a position below the shadowring before increasing the pressure within the chamber.
 16. An apparatusfor aligning a wafer on a support member in a vacuum chamber,comprising: a support member is positioned within the vacuum enclosurehaving a wafer receiving surface thereon; a shadow ring located withinthe vacuum chamber, the shadow ring comprising: an upper shield portiondefining a circular upper aperture therethrough, the upper aperturehaving a diameter that is slightly less than the outer diameter of thewafer; a lower portion extending from the upper shield portion having anannular cross section defining a frustoconical inner cavity; thediameter of the inner cavity decreases from a lower mouth aperture to anupper end; and the diameter of the upper end of the inner cavity isslightly greater than the outer diameter of the wafer; a gas supply influid communication with the vacuum chamber; a gas flow controller thatcontrols the flow of gas from the gas supply to the vacuum chamber andregulates the pressure within the vacuum chamber such that, after thewafer is positioned on the support member and before the wafer is raisedinto the shadow ring, the control member raises the pressure within thechamber.
 17. The apparatus of claim 16 wherein the control member raisesthe pressure within the vacuum chamber to about 1 Torr.
 18. Theapparatus of claim 16 wherein the control member raises the pressurewithin the vacuum chamber to a pressure between 1 Torr and 100 Torr. 19.The apparatus of claim 16 wherein the control member raises the pressurewithin the vacuum chamber to a pressure between 1 Torr and 10 Torr. 20.The apparatus of claim 16 wherein the control member raises the pressurewithin the vacuum chamber to a pressure that is at least about 1 Torrand below an operating pressure.
 21. The apparatus of claim 16 whereinthe difference between the diameter of the upper aperture in the uppershield portion and the outer diameter of the wafer is no greater than 5millimeters.
 22. The apparatus of claim 16 wherein the differencebetween the diameter of the upper aperture in the upper shield portionand the outer diameter of the wafer is no greater than 3 millimeters.23. The apparatus of claim 12 wherein the diameter of the upper end ofthe inner cavity is substantially equal to the outer diameter of thewafer.